This is a larger principle than emergencies. If you allow people to have a power they desire under condition x, they will endeavor to create, maintain, and extend condition x as much as possible. Condition x can be anything. If they really need some sort of emergency power, the proper way to reduce the incentive to abuse it is to simultaneously take away some powers while they have their special powers. For example, if a governor can declare an emergency and get special powers during that emergency, reduce his/her term by the length of time they have emergency powers. If their emergency declaration lasts two years, they only get two years as governor.